High force rotary actuator

ABSTRACT

An actuator includes a first stator with four first poles and a second stator with four second poles aligned with the four first poles. A permanent magnet is attached to the first stator and the second stator. Four moving armatures are positioned at terminal ends of the four first poles and the four second poles. Coils are wrapped around the first stator and the second stator. A controller selectively applies current to the coils to migrate flux created by the permanent magnet through selective poles of the first stator and the second stator and thereby alter the size of air gaps associated with the four moving armatures.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/086,650, filed Apr. 14, 2011, entitled High Force Rotary Actuator,which claims priority to U.S. Provisional Patent Application No.61/325,259, filed Apr. 16, 2010, the entire contents of all of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to devices to create motive forces.More particularly, this invention relates to a high force rotaryactuator.

BACKGROUND OF THE INVENTION

Most applications requiring high forces, such as robotic arms and lifts,use hydraulic actuators to develop the required force to move a payload.A hydraulic actuator system requires a gas or electric motor drivenpump, a reservoir, fluid lines and control valves operated either byhand or a microprocessor. It would be desirable to replace the hydraulicactuators and the ancillary components required to operate them in theseapplications with lightweight, high force electro-mechanical actuators.

Hydraulic actuators provide a mechanical advantage not unlike a gearboxor a transmission. With hydraulic pump and actuator systems the forcemultiplication or mechanical advantage is simply the ratio of the areaof the pump piston(s) to the area of the actuator piston. A hydraulicactuator is synonymous with a simple motor and gearbox since eachproduces a mechanical advantage. Therefore, electric motor gearboxsystems have been used to attempt to replace hydraulic actuators.Although high forces can be produced using an electric motor and gearboxthis comes at a sacrifice in output speed, which is proportional to thegearing ratio. The solution for increasing the response time is notsimply a matter of speeding up the motor feeding the gearbox since thetorque the motor produces typically reduces as speed is increased.Therefore, to increase response time the motor must not only run at ahigher speed, but must also increase in size. This increase in size notonly adds to system weight, but typically exceeds the allowablefootprint area for the actuator system.

Most linear and rotary actuators have an angular displacement as statorand armature poles come into and out of alignment. This angulardisplacement results in forces acting upon two axes. One axis representsthe force acting in the direction of the motion and the other axisrepresents the attractive force that attempts to pull the moving poleand the stationary pole together, which is perpendicular to thedirection of motion. The useable force to perform work is the vector sumof these forces. It would be desirable to provide techniques forimproved utilization of axial forces.

SUMMARY OF THE INVENTION

In one aspect, the disclosure is directed to a method comprising, in anactuator with (i) coils wrapped around a first plurality of polesdefining a first linear axis of a first stator and a second linear axisof the first stator orthogonal to the first linear axis, (ii) coilswrapped around a second plurality of poles of a second stator, thesecond plurality of poles forming aligned poles with the first pluralityof poles including first linear axis aligned poles and second linearaxis aligned poles, (iii) a permanent magnet attached to the firststator and the second stator, and (iv) a plurality of moving armaturesseparated from the aligned poles by a corresponding plurality of airgaps: selectively applying current to the coils to control a size of afirst air gap between a first moving armature and a first end of thefirst linear axis aligned poles to form a first linear motion and tocontrol a second size of a second air gap between a second movingarmature and a first end of the second linear axis aligned poles to forma second linear motion. The method further includes receiving outputforces of the first linear motion and the second linear motion at ashaft and translating the output forces to rotary motion of the shaft.

In another aspect, the disclosure is directed to a method comprising, inan actuator with coils wrapped around four first poles of a first statorand four second poles of a second stator aligned with the four firstpoles, a permanent magnet attached to the first stator and the secondstator, and four moving armatures positioned at terminal ends of thefour first poles and the four second poles: selectively applying currentto the coils to migrate flux created by the permanent magnet throughselective poles of the first stator and the second stator to selectivelyalter gap sizes associated with the four moving armatures.

In another aspect, the disclosure is directed to a method comprisingselectively applying current to first coils wrapped around a firstplurality of poles defining a first linear axis of a first stator and asecond linear axis of the first stator orthogonal to the first linearaxis and second coils wrapped around a second plurality of poles of asecond stator, the second plurality of poles forming aligned poles withthe first plurality of poles including first linear axis aligned polesand second linear axis aligned poles, to (i) control a size of a firstair gap between a first moving armature and a first end of the firstlinear axis aligned poles to form a first linear motion and (ii) controla second size of a second air gap between a second moving armature and afirst end of the second linear axis aligned poles to form a secondlinear motion. Output forces of the first linear motion and the secondlinear motion are received at a shaft and translated to rotary motion ofthe shaft.

In another aspect, the disclosure is directed to a method comprisingselectively applying current to coils wrapped around four first poles ofa first stator and four second poles of a second stator aligned with thefour first poles to migrate flux created by a permanent magnet throughselective poles of the first stator and the second stator to selectivelyalter gap sizes associated with four moving armatures positioned atterminal ends of the four first poles and the four second poles.

BRIEF DESCRIPTION OF THE FIGURES

The invention is more fully appreciated in connection with the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1A and 1B illustrate prior art rotary and linear actuator forcevectors; FIG. 1C illustrates a force vector formed in accordance with anembodiment of the invention.

FIG. 2 shows a permanent magnet with its flux divided through four poleslocated on upper and lower stators and through related armatures;

FIG. 3 shows how the permanent magnets flux division/multiplication isaffected by the air gap between the stator poles and their respectivearmatures.

FIGS. 4 and 5 show that using more than one permanent magnet inembodiments of the invention still results in physical and mathematicalbehavior of a single permanent magnet.

FIG. 6 shows how permanent magnets flux division/multiplication isaffected by the current flowing in the coils located on the statorpoles.

FIG. 7 shows a first set of slotted sliding members attached toarmatures that are opposite each other or displaced by 180 degrees toform a first linear actuator located about a central axis.

FIG. 8 shows a second set of slotted sliding members attached toarmatures that are opposite each other or displaced by 180 degrees butoffset by 90 degrees to form a second linear actuator located about acentral axis.

FIG. 9 shows a crank arm attached to an output shaft with bearings fixedfor rotation.

FIG. 10 shows the crank arm and output shaft placed into the actuator'sstator portions.

FIG. 11 shows the guide bearings that restrict the slotted slidersattached to the armatures to produce only linear motion along an axis.

FIG. 12 shows a sequence of motion for slotted sliders to interact withthe crank arm to translate linear motion into rotary motion.

FIG. 13 illustrates an exemplary control mechanism that may be used inaccordance with an embodiment of the invention.

FIG. 14 shows multiple actuators integrated into a system beingcontrolled by a main processor.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a stator pole 20 for a conventional linear actuator. Thestator pole 20 is composed from a magnetically soft material. A coil 21is wrapped around the stator pole 20. The coil 21 is energized tomagnetically attract the moving armature's pole 22, which is alsocomposed from a magnetically soft material. The forces from the magneticattraction have two main directions: one force 24 is perpendicular tothe direction of motion 23 and the second main force 25 acts at anangular displacement to the direction of motion 23. The vector sum ofthe forces 24 and 25 forms a resultant or net force 26 acting in thedirection of the motion 23, which will always be less than the sum ofthe forces 24 and 25. As the moving armature pole 22 moves intoalignment with the stator pole 20, the force acting perpendicular to thedirection of motion 24 becomes the dominant force, which reduces the netforce 26.

FIG. 1B shows a stator pole 27 for a conventional rotary actuatorcomposed from a magnetically soft material. A coil 29 is wrapped aroundthe stator pole 27. The coil 29 is energized to magnetically attract themoving rotor pole 28, which is also composed from a magnetically softmaterial. The forces from the magnetic attraction have two maindirections: one force 24 is perpendicular to the direction of motion 30and the second main force 25 acts at an angular displacement to thedirection of motion 30. The vector sum of the forces 24 and 25 forms aresultant or net force 26 acting in the direction of the motion 23. Thisnet force is always less than the sum of the forces 24 and 25. As themoving rotor pole 28 moves into alignment with the stator pole 27, theforce acting perpendicular to the direction of motion 30 becomes thedominant force, which reduces the net force 26.

FIG. 1C illustrates a single force 26 utilized in accordance with anembodiment of the invention. The single force 26 acts in the directionof the motion 34 when the coil 33 on the stator 31 is energized tomagnetically attract the moving armature 32. Consequently, the producedforce 26 is equal to the resultant or net force. Thus, the device ofFIG. 1C produces a relatively large force since no perpendicular forceis subtracted from the resultant force, as is the case in FIGS. 1A and1B. This desirable property holds as long as the air gap remainsrelatively small.

Also observe that control is through a single linear direction of theair gap. That is, the stator 31 and moving armature 32 are aligned in asingle linear direction (a vertical linear direction as shown in FIGS.1C). This stands in contrast to the case of FIGS. 1A and 1B where thereare two linear forces. For example, in FIG. 1A there is a verticalforce, as shown with arrow 24, and a force with a horizontal component,as shown with arrow 25.

FIG. 2 shows an upper armature 2 with a central section 2A in contactwith the North seeking pole of the permanent magnet 1 and four polesections 4, 6, 8, and 10 that protrude from the central section 2A.Poles 4 and 8 define a first linear axis, while poles 6 and 10 define asecond linear axis orthogonal to the first linear axis.

A lower armature 3 has a central section 3A in contact with thepermanent magnet's 1 South seeking pole and four pole sections 5, 7, 9,and 11 that protrude from the central section 3A. The poles on the upperarmature 2 are aligned with the poles on the lower armature 3 in thefollowing manner: pole 4 is aligned with pole 5, pole 6 is aligned withpole 7, pole 8 is aligned with pole 9 and pole 10 is aligned with pole11. Thus, there are aligned poles including first linear axis alignedpoles (4, 8, 5, 9) and second linear axis aligned poles (6, 10, 7, 11).

A moving armature is placed adjacent to each of the aligned poles in thefollowing manner: armature 13 is placed adjacent to poles 4 and 5,armature 14 is placed adjacent to poles 6 and 7, armature 15 is placedadjacent to poles 8 and 9, and armature 16 is placed adjacent to poles10 and 11. The flux 16 produced by the permanent magnet 1 traverses intothe central sections of the upper and lower armatures 2 and 3 anddivides through the four paths formed by the four upper and lower statorpoles through their respective armatures. Since the magnetic force ofattraction for one magnetic pole is given by the flux density in the airgap squared times the pole area divided by 0.5 times 1 Tesla/u_(o) andgiven that the flux from the permanent magnet is divided across fourpaths, then the force on any one of the armatures is 1/16 of the valueit would be if it were only acting across one armature.

The field intensity it requires to produce 1 Tesla of magnetic fluxdensity in the air gap is given by 1 Tesla/ u_(o) or 795,775 A/m.Therefore, to produce a flux density of 1 Tesla in the air gap between astator pole and the armature where the air gap length is 0.0064 m or0.25 inch, the required field intensity would be 5053 A/m. Since boththe upper and lower stators have a pole facing into their respectivearmature, this results in two air gaps per armature. Therefore, therequired field intensity would be 10,106 A/m to produce 1 Tesla in bothof the air gaps. If the air gap were reduced to 0.000254 m [0.010 inch]the required field intensity to produce 1 Tesla in both gaps would be404 A/m. Therefore, it could be stated that the reluctance in the airgap is reduced by 25 times.

In FIG. 3 if the air gap between armature 12 and the upper stator pole10 and the lower stator pole 11 were reduced to zero meters,substantially all of the flux from the permanent magnet 1 would traversethrough the upper and lower stator poles 10 and 11, respectively, andthrough armature 12. The force holding armature 12 to the stator polesincreases by about 16 times the force it was experiencing when the airgap was 0.0064 meters and the magnetic force attracting the remainingarmatures 13, 14 and 15 drops to essentially zero. Therefore, for achange in the air gap between any pair of stator poles and theirrespective armatures that share a single source of magnetic flux, inthis case permanent magnet 1, the resulting forces on the remainingarmatures divide or multiply accordingly.

In FIG. 4 if multiple magnets 35, 36 and 37 with their like magneticpoles aligned are added to the central sections of the upper stator 2and the lower stator 3 no increase in available magnetic flux occurssince the central sections of the stators still realize the sum of theflux they produce. For all practical purposes they still behave as asingle magnet or flux source which will still divide through thearmatures 12, 13 and 14 accordingly. The permanent magnets 35, 36 and 37in FIG. 4 are arced shaped magnets.

In FIG. 5 the permanent magnets 38 and 39 are rectangular in shape, butthis does not alter the fact that all flux, whether a single permanentmagnet or multiple permanent magnets is independent of shape andtherefore will still behave as a single flux source. The flux frompermanent magnets 38 and 39 will divide through the armatures 12, 13,14, and 15 accordingly. The available flux is determined by the totalarea of the pole face(s) of a permanent magnet and the length betweentheir poles, which produces the field intensity [A/m], which suggeststhat the greatest available flux will be produced by a single permanentmagnet residing in the central portions of the upper stator 2 and lowerstator 3, since it would have the greatest pole face area that would fitinto these central stator portions.

FIG. 6 shows control coils added to the poles 4, 6, 8, and 10 on theupper stator 2 and poles 5, 7, 9, and 11 on the lower stator 3. If acurrent is placed through the control coils 17 and 18, a magneticpolarity is produced, as shown on the upper stator pole 10 and on thelower stator pole 11 in FIG. 6. The magnetic polarity produced in thepole is opposite to that of the permanent magnet 1 contacting thecentral portion of that stator. The flux produced in the stator poles 10and 11 due to the current flowing through the control coils 17 and 18will couple with the flux of permanent magnet 1 and this combined fluxwill traverse through armature 12. Since the magnetic poles of thecontrol coils 17 and 18 facing into the central portion of the upper andlower stators, respectively 2 and 3, only have a path through thepermanent magnet 1 back to their magnetic poles facing toward andthrough armature 16, therefore if the flux produced by control coils 17and 18 is greater than the portion of the permanent magnet flux alreadytraversing through stator poles 11 and 10 a portion of the permanentmagnet 1 flux traversing through stator poles 5, 6, 7, 8, and 9 andtheir respective armatures 13, 14, and 15 will be diverted or redirectedthrough the stator poles 10 and 11 and its respective armature 12 untilthe amount of this redirected flux equals the flux produced by thecontrol coils 17 and 18. As the current is increased through the controlcoils 17 and 18, the flux 16 traversing through the stator poles 10 and11 and through armature 12 increases and the flux traversing theremaining stator poles and their respective armatures decreases, whichresults in a stronger attracting force on armature 12 and a weaker forceon the remaining stator poles and their respective armatures. If thecurrent is increased in the control coils 17 and 18 to where the fluxproduced in the stator poles 10 and 11 and through armature 12 equalsthe entire flux produced by the permanent magnet 1 all of the flux 16from permanent magnet 1 will traverse through the stator poles 10 and 11and armature 12, but in this case will increase the force on armature 12to a value much greater than 16 times, as discussed in connection withFIG. 3. This is due to the fact that the field intensity of the controlcoil adds to the field intensity of the permanent magnet, which willincrease the flux density in the air gap between armature 12 and statorpoles 10 and 11.

FIGS. 3 and 6 demonstrate that the resultant force on any on thearmatures 12, 13, 14, and 15 is controlled by both the length of theirindividual air gaps between the armatures 12, 13, 14, and 15 and theirrespective stator poles 4, 5, 6, 7, 8, 9, 10, and 11 and the currentflowing through the control coils wound on all of the stator poles. Bycontrolling the current through various control coils the forces on allof the armatures can be kept balanced or can be increased or decreasedindependent of air gap length.

FIG. 7 shows the formation of a first linear actuator by attaching oneend of a slotted slider 41 and a nonmagnetic spacer 43 to armature 13and the opposite end of the slotted slider 41 and a nonmagnetic spacer43 to armature 15. Armatures 13 and 15 remain in alignment with theirassociated stator poles 4, 5, 8, and 9 but armatures 13 and 15 must movein unison with one another or as the air gap between one pair of statorpoles and its associated armature increases the air gap between theopposite pair of stator poles and its associated armature must decreaseand vice versa.

FIG. 8 shows the formation of a second linear actuator by attaching oneend of a slotted slider 44 and a nonmagnetic spacer 43 to armature 12and the opposite end of the slotted slider 41 and a nonmagnetic spacer43 to armature 14. Armatures 12 and 14 remain in alignment with theirassociated stator poles 6, 7, 10, and 11 but armatures 12 and 14 mustmove in unison with one another or as the air gap between one pair ofstator poles and its associated armature increases the air gap betweenthe opposite pair of stator poles and its associated armature mustdecrease and vice versa.

FIG. 9 shows a crank arm 46 attached to an output shaft 48 with bearings47 mounted on the output shaft 48 to allow rotation of both the crankarm 46 and output shaft 48. This crank arm and shaft act as motiontranslator between the two linear actuators described in FIGS. 7 and 8from linear motion to rotary motion. It should be noted that other meansfor the translation from linear motion to rotary motion exist and thismethod is shown as one example and where the motion translation means isnot pertinent to the invention disclosed herein.

FIG. 10 shows the crank arm 46, bearings 47, and output shaft 48inserted into the stator assembly comprised of the upper stator 2 andlower stator 3 and permanent magnet 1 allowing the crank arm to rotatein the central axis of the stator assembly.

FIG. 11 shows guide bearings 52 and 55 placed adjacent to one side ofthe slotted slider 41 and guide bearings 53 and 54 placed adjacent tothe opposite side of the slotted slider 41 where these guide bearingsconstrain the slotted slider 41 to move along the axis 57. FIG. 11 alsoshows guide bearings 50 and 51 placed adjacent to one side of theslotted slider 44 and guide bearings 49 and 52 placed adjacent to theopposite side of the slotted slider 44 where these guide bearingsconstrain the slotted slider 44 to move along the axis 58. The slots inthe slotted sliders 41 and 44 are placed over the crank arm 46, whichmaintains a specific relationship between the linear position betweenthe slotted sliders 41 and 44 and the angular position of the crank arm46.

This linear to angular relationship between the slotted sliders 41 and44 and the crank arm 46 is shown in FIG. 12. If the control coils aresequentially energized in an ordered manner, the slots in each of theslotted sliders will impart a continuous rotary motion and a high forceto the output shaft 48. FIG. 12 illustrates a sequence of clock-wiselinear transpositions beginning at (a) and ending at (h), which producesrotary motion. Motion from (a) to (c) is linear in a first horizontaldirection, while motion from (c) to (e) is linear in a first verticaldirection, orthogonal to the first horizontal direction. Motion from (e)to (g) is linear in a second horizontal direction opposite the motion of(a) through (c). Motion from (g) to (h) is linear in a second verticaldirection opposite the motion from (c) through (e).

If a rotating mechanical input power is applied to the output shaft 48the crank shaft will move the slotted sliders and their attachedarmatures in a manner to create a flux swing through each of the controlcoils generating electrical energy. Since this novel actuator is alsocapable of acting as a generator it lends itself well to four quadrantcontrol where coasting could generate electrical power that can beshunted for braking or to recharge batteries.

The control coils are sequentially energized by a controller. FIG. 13illustrates a controller that may be used in accordance with anembodiment of the invention. The controller sequentially energizes thecontrol coils 59 through 66 in an ordered manner through the firing ofthe power switches 67 through 74. The power switches are connected tobus 77, which has a current and sequence controller 78 governed by aprocessor 81. Processor 81 additionally acquires information from acurrent sensor 79 and 80 and slotted slider position sensors 82.

FIG. 14 shows a polarity of the new high force actuators 83 integratedinto a system requiring several actuators under the control of a mainprocessor 85. The main processor 85 communications with actuator andcontrollers 83 via a bus 84.

Thus, the invention includes one or more linear actuators placed arounda central axis. Output forces of the linear actuators are summed andtranslated to rotary motion, but an individual linear actuator onlyproduces force along its axis of motion. Therefore, the sum of all ofthe forces produced by the one or more linear actuators appears at theoutput of the actuator.

The mechanism of the invention creates high linear forces over shortdistances. Advantageously, the sum of a set of linear forces istranslated to rotary motion, resulting in high forces with high responsetimes. This is accomplished without hydraulics or a transmissionstructure. Thus, the invention may be implemented as a light weight highpower density actuator. The actuator is controlled with permanent magnetflux division and multiplication modes.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed; obviously, many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications, they thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the following claims and their equivalents define thescope of the invention.

What is claimed is:
 1. A method comprising: in an actuator comprising:coils wrapped around a first plurality of poles defining a first linearaxis of a first stator and a second linear axis of the first statororthogonal to the first linear axis; coils wrapped around a secondplurality of poles of a second stator, the second plurality of polesforming aligned poles with the first plurality of poles including firstlinear axis aligned poles and second linear axis aligned poles; apermanent magnet between the first stator and the second stator; and aplurality of moving armatures separated from the aligned poles by acorresponding plurality of air gaps; selectively applying current to thecoils to control a size of a first air gap between a first movingarmature and a first end of the first linear axis aligned poles to forma first linear motion and control a second size of a second air gapbetween a second moving armature and a first end of the second linearaxis aligned poles to form a second linear motion; and receiving outputforces of the first linear motion and the second linear motion at ashaft and translating the output forces to rotary motion of the shaft.2. The method of claim 1 further comprising applying current to thecoils by a controller to decrease an air gap between a controlled movingarmature and controlled aligned poles to produce a divided force betweenremaining moving armatures and remaining aligned poles.
 3. The method ofclaim 1 further comprising applying current to the coils by a controllerto increase an air gap between a controlled moving armature andcontrolled aligned poles to produce a multiplied force between remainingmoving armatures and remaining aligned poles.
 4. The method of claim 1further comprising receiving the first linear motion at a first slottedslider positioned along the first linear axis and receiving the secondlinear motion at a second slotted slider positioned along the secondlinear axis to manipulate a crank arm of the actuator.
 5. The method ofclaim 1 further comprising sequentially applying current to the coils inan ordered manner to impart a continuous rotary motion on the shaft. 6.The method of claim 1 further comprising sequentially applying currentto the coils in a clockwise manner to impart a continuous rotary motionon the shaft.
 7. A method, comprising: in an actuator comprising: coilswrapped around four first poles of a first stator and four second polesof a second stator aligned with the four first poles; a permanent magnetbetween the first stator and the second stator; and four movingarmatures positioned at terminal ends of the four first poles and thefour second poles; selectively applying current to the coils to migrateflux created by the permanent magnet through selective poles of thefirst stator and the second stator to selectively alter gap sizesassociated with the four moving armatures.
 8. The method of claim 7further comprising dividing the flux between the selective poles andremaining poles.
 9. The method of claim 7 further comprising multiplyingthe flux between the selective poles and remaining poles.
 10. The methodof claim 7 further comprising applying current to the coils to move eachmoving armature of the four moving armatures in a respective singlelinear direction.
 11. The method of claim 7 further comprisingtranslating linear forces created by the moving armatures at a shaft toa rotary force.
 12. The method of claim 7 further comprising engaging acrank arm by a first slotted slider and a second slotted slider.
 13. Themethod of claim 7 further comprising sequentially applying current tothe coils in an ordered manner to impart a continuous rotary motion onthe shaft.
 14. The method of claim 7 further comprising sequentiallyapplying current to the coils in a clockwise manner to impart acontinuous rotary motion on the shaft.
 15. A method comprising:selectively applying current to (i) first coils wrapped around a firstplurality of poles defining a first linear axis of a first stator and asecond linear axis of the first stator orthogonal to the first linearaxis and (ii) second coils wrapped around a second plurality of poles ofa second stator, the second plurality of poles forming aligned poleswith the first plurality of poles including first linear axis alignedpoles and second linear axis aligned poles to: control a size of a firstair gap between a first moving armature and a first end of the firstlinear axis aligned poles to form a first linear motion; and control asecond size of a second air gap between a second moving armature and afirst end of the second linear axis aligned poles to form a secondlinear motion; and receiving output forces of the first linear motionand the second linear motion at a shaft and translating the outputforces to rotary motion of the shaft.
 16. A method, comprising:selectively applying current to coils wrapped around four first poles ofa first stator and four second poles of a second stator aligned with thefour first poles to migrate flux created by a permanent magnet throughselective poles of the first stator and the second stator to selectivelyalter gap sizes associated with four moving armatures positioned atterminal ends of the four first poles and the four second poles.